The present invention concerns a twin fabric gap blade type forming section of a papermaking machine and an apparatus for improving sheet formation therein.
The modern two fabric (or twin wire) papermaking machines in which the papermaking stock or furnish is delivered from the headbox slice lip into the gap between two converging forming fabrics is often referred to as a gap former. In a gap former, one of the two forming fabrics is supported either by wrapping it around a rotating roll surface, or by wrapping it over the fabric support surfaces comprised of a series of blades mounted on a supporting stationary shoe. There are two types of gap formers.
In the first type of gap former, referred to as a gap roll former, initial impingement of the stock jet is onto a fabric supported on a roll surface; this arrangement provides for constant pressure drainage, with excellent fiber retention, excellent tolerance to impingement angle variations, but poor sheet formation due to the absence of the beneficial pressure pulses provided by blade edges.
In the second type of gap former, known as a gap blade former, the initial impingement of the stock jet is onto a fabric which is supported by a series of blade surfaces. The strong pressure pulses generated by fabric deflection at the edges of each blade surface results in excellent sheet formation, but poor fiber retention and an almost zero tolerance for variations in the angle of impingement of the stock jet from the headbox slice.
Recent advances in two fabric forming technology have included the introduction of stationary curved impingement shoe technology, for example, as shown in US 2003/0173048 to Buchanan et al. The stationary curved impingement shoe, which is partially wrapped by both fabrics, replaces the roll or blade impingement zones described above. The curved impingement shoe provides an impingement zone that dewaters the sheet in a manner similar to that obtained with a curved roll surface as in the gap roll former, but combined with a series of much gentler pressure pulses than are possible with the traditional bladed stationary shoe as in the gap blade former. In gap formers equipped with the stationary curved impingement shoe such as those described in the present invention, a fabric lead-in roll is present immediately upstream to the impingement shoe.
Following the initial impingement zone in any of these aforementioned roll, blade or impingement type gap former arrangements, the two forming fabrics together with the papermaking furnish sandwiched between them pass over a stationary bladed forming shoe for further dewatering and formation enhancing pressure pulses provided by the blade edges. This formation shoe is most often placed on the inside run of the opposite fabric to that on which the impingement occurs to provide for a balancing of the drainage. It is also known to use adjustable pressurized blades that can be loaded by means of hydraulic or air pressure so as to push them into the fabrics, such as are described by U.S. Pat. No. 6,361,657, that are mounted opposite the stationary bladed forming shoe so that the individual pressurized blades can press into the two fabrics (with the stock sandwiched between them) in the gaps between the stationary blades on the forming shoe.
The adjustable pressurized blades provide a further degree of adjustment and control over the magnitude of the pressure pulses occurring at each stationary blade edge. These adjustable blades are capable of providing dramatic improvements in sheet quality, but only if the sheet consistency entering this portion of the forming section is within a very narrow range that permits beneficial fiber realignment. If the consistency is too high or too low, then the use of these blades can be detrimental to sheet quality, rather than providing an improvement.
The consistency of the embryonic sheet entering the opposed blade section is determined by many factors, including: basis weight of the product being manufactured, consistency of the stock exiting the headbox slice, composition of the stock and, most importantly, the amount of drainage occurring in the initial impingement zone. The drainage occurring in the initial impingement zone, whether it be in a gap roll, gap blade or curved impingement shoe forming arrangement, is also dependant on the above mentioned factors as well as the fabric tension and the angle of wrap (i.e. the amount of the curved surface over which the two fabrics wrap in the impingement zone). It is well known that the angle of wrap in the impingement zone is fixed by machine geometry and therefore the consistency entering the opposed blade section will change if any of the operational variables fluctuate, and consequently paper quality will suffer.
The headbox is a massive object and, although so-called slice lips are provided which may be used to direct the stream of stock being ejected from the headbox onto a desired location on the impingement shoe, fine control of these lips is imprecise and cumbersome. It is very difficult in practice to precisely direct the stock jet onto the impingement shoe so that it impacts at a specific location with precision.
U.S. Pat. No. 4,523,978 discloses the use of a forming shoe in a twin wire former. The forming shoe is located downstream of the initial single wire portion of the forming section so that it only acts after some degree of felting of the embryonic paper web has taken place and the fibers are no longer able to move relative to each other. The position or attitude of the forming shoe is adjustable in order to adjust the dewatering capability of the forming section as well as web formation. This arrangement is not indicated as effecting fiber distribution in the embryonic paper web.